Polyesters, particularly unsaturated polyesters, present attractive prospects as substitutes for metal parts in many applications, such as automotive grills and other decorative pieces. Polyesters offer basically good molding characteristics, economy, and lightweight, though they have lacked adequate impact strength, particularly when combined with reinforcing agents and/or fillers.
Improvements in use of unsaturated polyester compositions have been attempted by incorporating rubbers into the composition. Various liquid, semi-solid, or solid rubbers have been mixed with polyesters to produce thermosetting compositions with better impact strength. However, rubbers generally have not been compatible with polyesters, resulting in non-uniform mixtures, and sometimes delamination has occurred.
Problems with compatibility have been substantially solved by the incorporation with the unsaturated polyester of rubbery carboxy-polymers of conjugated dienes into a composition further containing a vinyl monomer, usually further with a reinforcing agent and/or filler. The carboxy-containing rubbery polymers have proven highly compatible with the unsaturated polyester, mix well, and result in compositions (compounds) for both bulk molding (BMC) and sheet molding (SMC) which provide uniformity, avoid delamination, and exhibit improved pigmentability while yet retaining good physical properties including good impact strength. The carboxy-terminated rubbery polymers also act as modifiers which reduce mold shrinkage frequently attendant to polyester molding.
Still, a continuing recognized problem in compression molded thermoset polyester parts, from both bulk molding compounds (BMC) and from sheet molding compounds (SMC), is that the surface appearance and smoothness of the parts simply are not anywhere equivalent to or equal to that obtainable from metal. This general problem of deficient surface appearance also is true of plaques molded from glass-reinforced polyester compositions containing a carboxy-terminated rubber as modifier.
Needed is a polyester molding composition of good strength, moldability, and lightweight, as demanded in many applications for metal substitution, but which exhibits the good surface "look" to which the customer is accustomed.